Technique for Assigning Pilot Signals to User Equipments

ABSTRACT

A technique for assigning pilot signals to user equipments is described. The user equipments access a radio base station that provides radio access. As to a method aspect of the technique, an access burst including initial pilot signals is received from the user equipments, wherein at least two of the user equipments apply simultaneously the same initial pilot signal. A multicast channel to the at least two user equipments is estimated based on the received access burst. A combined signal power received at the radio base station is computed for the at least two user equipments based on the estimated multicast channel. A message to the at least two user equipments is sent using the multicast channel. The message is indicative of the combined signal power and assigns different pilot signals to the at least two user equipments, wherein the assignment depends for each of the at least two user equipments on the combined signal power and a signal power received at the respective user equipment.

This application is a continuation of prior U.S. patent application Ser.No. 14/429,534, filed 19 Mar. 2015, which was the National Stage ofInternational Application No. PCT/SE2015/053737, filed 23 Feb. 2015, thedisclosures of all of which are incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The present disclosure generally relates to a technique for assigningpilot signals to user equipments. More specifically, and withoutlimitation, methods and devices are provided for assigning pilot signalsto user equipments when accessing a radio base station that providesradio access by means of a plurality of transceiving antennas.

BACKGROUND

Massive Multiple-Input Multiple-Output (massive-MIMO) radio transmissionis a technique for wireless access, e.g., in telecommunications networkslike or beyond Long Term Evolution (LTE). Hundreds of phase-coherentlyoperating base station antennas serve many tens of user equipments onthe same time-frequency resource.

Each user equipment needs to transmit its own pilot signal (alsoreferred to as reference signal) in a coherence interval. The pilotsignal allows the radio base station (e.g., an eNodeB in an LTE network)to measure Channel State Information (CSI) for the coherence interval.The massive-MIMO transmission uses the CSI for closed-loop beamforming,so that the number of user equipments that can be served per radio basestation is significantly increased. However, the increased user densityleads to an increase in the rate of user equipments accessing the radiobase station for the first time, e.g., because the user equipment hasbeen turned on or the user equipment has entered the coverage area ofthe radio base station. Furthermore, the closed-loop beamforming inmassive-MIMO transmissions allows for high-mobility channels (e.g., foruser equipments located in a high-speed train), which further adds tothe rate of first-time accesses at the radio base station. For thefirst-time access, the user equipments select their pilot signals, e.g.,randomly. The same pilot signal may be selected by different userequipments in the same coherence interval for the first-time access,since the number of different pilot signals is limited while the numberof accessing user equipments is increasing. Hence, access bursts withcolliding pilot signals can be expected to become more frequent.

Conventionally, user equipments transmitting the same pilot signal haveto reattempt until their access bursts are collision-free, which causesdelay and occupies radio resources. Increasing the length of the pilotsignals would increase the number of distinct pilot signals. However,the longer the pilot signals the more radio capacity is occupied andunavailable for payload data.

SUMMARY

Accordingly, there is a need for a technique that, in at least somesituations, efficiently assigns different pilot signals to differentuser equipments.

As to one aspect, a method of assigning pilot signals to user equipmentsaccessing a radio base station that provides radio access is provided.The method comprises a step of receiving an access burst includinginitial pilot signals from the user equipments, wherein at least two ofthe user equipments apply simultaneously the same initial pilot signal;a step of estimating a multicast channel to the at least two userequipments based on the received access burst; a step of computing acombined signal power received at the radio base station for the atleast two user equipments based on the estimated multicast channel; anda step of sending a message to the at least two user equipments usingthe multicast channel, wherein the message is indicative of the combinedsignal power and assigns different pilot signals to the at least twouser equipments, wherein the assignment depends for each of the at leasttwo user equipments on the combined signal power and a signal powerreceived at the respective user equipment.

At least some embodiments of the technique allow assigning differentpilot signals to the at least two user equipments in that the assignmentdepends on the received signal powers. The at least two user equipmentsmay be distinguished based on respectively received signal powers. Sameor other embodiments of the technique can efficiently assign allavailable pilot signals. E.g., embodiments can assign few remaining(i.e., not yet assigned pilot signals) without collisions or accessretries, optionally including the assignment of the very last remainingpilot signal. Access reattempts, particularly unsuccessful reattempts,can be avoided in at least some situations.

The radio base station may be part of a telecommunications network. Theradio base station may be part of a Radio Access Network for mobiletelecommunication. The user equipments, e.g., the at least two userequipments, may comprise mobile, portable and/or stationary terminals.

The method may be performed by the radio base station. The access burstmay be received on a Random Access Channel (RACH) of the radio basestation. The pilot signal may also be referred to as a pilot sequence,preamble sequence or RACH preamble. The number of different pilotsignals may be a power of two, e.g., 2⁶=64.

The pilot signal may also be referred to as a reference signal.Estimating the multicast channel may include at least one of determiningChannel State Information (CSI) for the multicast channel anddetermining a precoder for the multicast channel. Using the multicastchannel may include simultaneously beamforming from the plurality oftransceiving antennas towards the at least two user equipments.

The received signal power may encompass a sent power (or transmit power)p and a channel gain (or link attenuation) β. The received signal powermay include a product of link attenuation β and transmit power p. Thechannel gain β may include an attenuation of a radio link between theradio base station and the respective user equipment.

The signal power received at the user equipments A and B may be equal tothe products p_(A)·β_(A) and p_(B)·β_(B), respectively. The combinedsignal power received at the radio base station may be equal to the sumof the signal powers received at the respective user equipments, e.g.,p_(A)·β_(A)+p_(B)·β_(B).

The combined signal power received at the radio base station and thesignal power received at the respective user equipment may relate to thesame coherence interval or the same resource block. The combined signalpower received at the radio base station and the signal power receivedat the respective user equipment may be related due to channelreciprocity. The signal power received at the respective user equipmentmay be computed based on the message as received at the respective userequipment or based on a broadcast message from the radio base station asreceived at the respective user equipment. The broadcast message mayinclude a beacon frame. The access burst may be received and the messageor the broadcast message may be sent within the same coherence intervalor the same resource block.

The at least two user equipments may include a first user equipment Aand a second user equipment B different from the first user equipment A.The message may assign at least one first pilot signal s_(A) to thefirst user equipment A, if twice the signal power received at the firstuser equipment A is greater than the combined signal power. The messagemay assign at least one second pilot signal s_(B) (which is differentfrom each of the at least one first pilot signal s_(A)) to the seconduser equipment B, if twice the signal power received at the second userequipment B is less than the combined signal power.

The pilot signals may be assigned so that the first pilot signal isassigned to the one of the user equipments which contribution to thecombined signal power is greater than the contribution of the other ofthe two user equipments. The second pilot signal may be assigned to theone of the user equipments which contribution to the combined signalpower is less than the contribution of the other of the two userequipments.

For any number of the at least two user equipments, the pilot signalsmay be assigned in the order of the signal power contribution of therespectively assigned one of the at least two user equipments to thecombined signal power.

The message may assign a first set S_(A) of pilot signals to the firstuser equipment and a second set S_(B) of pilot signals disjoint from thefirst set to the second user equipment. The first set S_(A) may include,or may be defined by, the at least one first pilot signal s_(A). Thesecond set S_(B) may include, or may be defined by, the at least onesecond pilot signal s_(B). The first user equipment A and the seconduser equipment B may select one first pilot signal s_(A) and one secondpilot signal s_(B) out of the first set S_(A) and the second set S_(A),respectively. The selection may be based on a (pseudo) random number oran identifier of the respective user equipment. A user-specificidentifier (e.g., stored on a SIM card) may be mapped onto the pilotsignals in the respective set.

The initial pilot signals from the at least two user equipments may bereceived in the same coherence interval and/or the same resource block.The initial pilot signals from the at least two user equipments mayoverlap in time and frequency.

The radio base station may determine that the at least two userequipments use or apply the same initial pilot signal, if the receivedaccess burst is not decodable and a signal strength of the access burstexceeds a threshold value. The access burst may be not decodable as toinformation specific for each of the at least two user equipments. Thesignal strength may include at least one of a signal-to-noise ratio andthe combined signal power.

The radio base station may provide the radio access by means of aplurality of transceiving antennas. The computation may include asummation over the plurality of transceiving antennas.

The summation may include squared absolute values of antenna componentsfor the multicast channel. The antenna components may be related to thetransceiving antennas, respectively. The plurality of transceivingantennas may include more than 10, e.g., more than 100 or 200, antennasat the radio base station. The radio base station may be configured fora massive Multiple-Input Multiple-Output (MIMO) communication with theuser equipments. By way of example, the MIMO system (e.g., according toLTE) may include 8 antenna ports and more than 100 antennas. A number ofuser equipments served by the radio base station may be equal to orgreater than 10, e.g., in the range of 40 to 50.

A further access burst may be received from each of the at least twouser equipments after sending the message. The further access burst mayinclude the respectively assigned pilot signal. Channel estimation maybe performed for each of the at least two user equipments based on therespectively received further access burst.

The radio access may include a Time Division Duplex (TDD) transmission.The pilot signals may be configured for reverse channel estimation. Theradio base station and the user equipments may form a TDD MIMO system.Channel reciprocity may be used to train a reverse link. Channelreciprocity may be used at the radio base station to obtain the channelestimate for the multicast channel to the at least two user equipmentsbased on the same initial pilot signal and/or to obtain the channelestimates for the individual channels to each of the at least two userequipments.

As to another aspect, a method of assigning pilot signals to userequipments accessing a radio base station that provides radio access isprovided. The method comprises a step of sending an access burstincluding an initial pilot signal from a user equipment to the radiobase station, wherein at least one other of the user equipments appliessimultaneously the same initial pilot signal; a step of receiving amulticast message from the radio base station, wherein the multicastmessage is indicative of a combined signal power received at the radiobase station from the user equipment and the at least one other userequipment; a step of estimating a channel to the radio base station; astep of computing a signal power received at the user equipment based onthe estimated channel; and a step of determining a pilot signal assignedto the user equipment, wherein the assignment depends on the combinedsignal power indicated by the multicast message and the signal powerreceived at the user equipment.

The method may be performed by the user equipment. The access burst maybe sent when the user equipment switches from a Radio Resource Control(RRC) idle state to an RRC connected state, when the user equipmentloses synchronization with the radio base station (e.g., during uplinkdata transfer to the radio base station) or when the user equipmentre-establishes the RRC connection (e.g., upon detecting a radio linkfailure or handover failure).

The channel may be estimated based on the multicast message as receivedat the user equipment or based on a broadcast message as received at theuser equipment from the radio base station.

A first pilot signal may be assigned to the user equipment, if twice thesignal power received at the user equipment is greater than the combinedsignal power. A second pilot signal (that is different from the firstpilot signal) may be assigned to the user equipment, if twice the signalpower received at the user equipment is less than the combined signalpower.

A further access burst including the assigned pilot signal may be sentto the radio base station.

Any feature disclosed in the context of the one method aspect may beimplemented in the context of the other method aspect. Any stepcorresponding to a step disclosed in the context of the one methodaspect may also be performed in the context of the other aspect.

As to a further aspect, a computer program product is provided. Thecomputer program product comprises program code portions for performingany one of the steps of the method aspects disclosed herein when thecomputer program product is executed by one or more computing devices.The computer program product may be stored on a computer-readablerecording medium. The computer program product may also be provided fordownload via a data network, e.g., the telecommunications network and/orthe Internet.

As to a still further aspect, a network node for assigning pilot signalsto user equipments accessing a radio base station that provides radioaccess is provided. The network node comprises: an interface configuredto communicate with the radio base station; and at least one processorcoupled to the interface and configured to trigger the steps of any oneof the one method aspect.

As to a still further aspect, a network node for assigning pilot signalsto user equipments accessing a radio base station that provides radioaccess is provided. The network node comprises: an interface configuredto communicate with the user equipments; and at least one processorcoupled to the interface and configured to trigger the steps of any oneof the another method aspect.

The network nodes may be implemented in a distributed network, e.g., theInternet.

According to one hardware aspect, a device for assigning pilot signalsto user equipments accessing a radio base station that provides radioaccess is provided. The device comprises a receiving unit adapted toreceive an access burst including initial pilot signals from the userequipments, wherein at least two of the user equipments applysimultaneously the same initial pilot signal; an estimating unit adaptedto estimate a multicast channel to the at least two user equipmentsbased on the received access burst; a computing unit adapted to computea combined signal power received at the radio base station for the atleast two user equipments based on the estimated multicast channel; anda sending unit adapted to send a message to the at least two userequipments using the multicast channel, wherein the message isindicative of the combined signal power and assigns different pilotsignals to the at least two user equipments, wherein the assignmentdepends for each of the at least two user equipments on the combinedsignal power and a signal power received at the respective userequipment.

According to another hardware aspect, a device for assigning pilotsignals to user equipments accessing a radio base station that providesradio access is provided. The device comprises a sending unit adapted tosend an access burst including an initial pilot signal from a userequipment to the radio base station, wherein at least one other of theuser equipments applies simultaneously the same initial pilot signal; areceiving unit adapted to receive a multicast message from the radiobase station, wherein the multicast message is indicative of a combinedsignal power received at the radio base station from the user equipmentand the at least one other user equipment; an estimating unit adapted toestimate a channel to the radio base station; a computing unit adaptedto compute a signal power received at the user equipment based on theestimated channel; and a determining unit adapted to determine a pilotsignal assigned to the user equipment, wherein the assignment depends onthe combined signal power indicated by the multicast message and thesignal power received at the user equipment.

Any one of the units of the devices, or a further dedicated unit, may beadapted to perform any one of the steps disclosed in the context of themethod aspect. Furthermore, the devices may comprise any featuredisclosed in the context of the method aspect.

As to a further aspect, a network node for assigning pilot signals touser equipments accessing a radio base station that provides radioaccess is provided. The network node comprises: an access burstreception module for receiving an access burst including initial pilotsignals from the user equipments, wherein at least two of the userequipments apply simultaneously the same initial pilot signal; a channelestimation module for estimating a multicast channel to the at least twouser equipments based on the received access burst; a signal powercomputation module for computing a combined signal power received at theradio base station for the at least two user equipments based on theestimated multicast channel; and a message send module for sending amessage to the at least two user equipments using the multicast channel,wherein the message is indicative of the combined signal power andassigns different pilot signals to the at least two user equipments,wherein the assignment depends for each of the at least two userequipments on the combined signal power and a signal power received atthe respective user equipment.

As to a still further aspect, a mobile terminal for assigning pilotsignals to user equipments accessing a radio base station that providesradio access is provided. The mobile terminal comprises: An access burstsend module for sending an access burst including an initial pilotsignal from a user equipment to the radio base station, wherein at leastone other of the user equipments applies simultaneously the same initialpilot signal; a message reception module for receiving a multicastmessage from the radio base station, wherein the multicast message isindicative of a combined signal power received at the radio base stationfrom the user equipment and the at least one other user equipment; achannel estimation module for estimating a channel to the radio basestation; a signal power computation module for computing a signal powerreceived at the user equipment based on the estimated channel; and apilot signal determination module for determining a pilot signalassigned to the user equipment, wherein the assignment depends on thecombined signal power indicated by the multicast message and the signalpower received at the user equipment.

The modules may be implemented by a computer program stored in memorycoupled to a processor for running the computer program.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present disclosure is described in more detailwith reference to exemplary embodiments illustrated in the drawings,wherein:

FIG. 1 is a block diagram schematically illustrating an embodiment of anetwork.

FIG. 2 is a block diagram schematically illustrating an embodiment of adevice.

FIG. 3 is a block diagram schematically illustrating an embodiment of adevice for assigning pilot signals to user equipments, which isimplementable at any one of the user equipments of FIG. 1.

FIG. 4 is a flowchart illustrating embodiments of the method steps.

FIG. 5 is a flowchart illustrating embodiments of the method steps.

FIG. 6 is a flowchart illustrating embodiments of the method steps.

FIG. 7 is a diagram schematically illustrating an embodiment of amulticast channel used for an implementation of the methods of FIGS. 4and 5.

FIG. 8 is a diagram schematically illustrating an embodiment of anetwork node.

FIG. 9 is a diagram schematically illustrating an embodiment of a mobileterminal.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as specific networkenvironments and specific transmission scenarios in order to provide athorough understanding of the technique disclosed herein. It will beapparent to one skilled in the art that the technique may be practicedin other embodiments that depart from these specific details. Moreover,while the following embodiments are primarily described for a mobiletelecommunications network operating next-generation antenna arrays, itwill be readily apparent that the technique described herein may also beimplemented in other mobile and stationary communication networks,including Wireless Local Area Networks (WLAN) according to IEEE 802.11standards, Worldwide Interoperability for Microwave Access (WiMAX)networks according to IEEE 802.16 standards, Global System for MobileCommunications (GSM) networks, Universal Mobile TelecommunicationsSystem (UMTS) networks, Long Term Evolution (LTE) networks andLTE-Advanced networks.

Moreover, those skilled in the art will appreciate that the functions,steps and units explained herein may be implemented using softwarefunctioning in conjunction with a programmed microprocessor, anApplication Specific Integrated Circuit (ASIC), a Field ProgrammableGate Array (FPGA), a Digital Signal Processor or a general purposecomputer, e.g., including an Advanced RISC Machine (ARM) processor. Itwill also be appreciated that, while the following embodiments areprimarily described in context with methods and devices, the inventionmay also be embodied, e.g., according to modules, in a computer programproduct as well as in a system comprising a computer processor andmemory coupled to the processor, wherein the memory is encoded with oneor more programs that may perform the functions, steps and implement theunits or modules disclosed herein.

FIG. 1 schematically illustrates a telecommunications network 100 as anexemplary environment for implementing the technique. Thetelecommunications network 100 comprising at least one radio basestation (RBS) 102. The telecommunications network 100 further comprisesuser equipments (UEs) 106. The RBS includes a plurality of transceivingantennas 104. The RBS 102 provides radio access by means of thetransceiving antennas 104. The UEs 106 access the RBS 102. Two or moreUEs 106 are within range for radio access to the RBS 102.

The number of antennas 104 and UEs 106 may deviate from those shown inFIG. 1. E.g., the technique is implemented using 40 to 50 antennasand/or less than 10 UEs.

In the context of an LTE implementation, the RBS 102 is also referred toas an eNodeB or eNB.

While different base station sites may cooperate for serving one or moreof the UEs, this is not necessary for the technique. For clarity, andwithout limitation, the technique is described with reference to thetelecommunications network 100 in FIG. 1, which provides radio access bymeans of the RBS 102.

For communication with the RBS 102 via a Multiple-Input Multiple Output(MIMO) channel, each UE 102 needs its own pilot signal. Different UEs106 have to have different pilot signals, e.g., for establishing a RadioResource Control (RRC) connected state with the RBS 102. The pilotsignals should be mutually orthogonal. In other words,cross-correlations of one pilot signal with the pilot signals used byeveryone else should be substantially lower than the peakautocorrelation of the pilot signal.

If τ samples are allocated to the pilot signal, there are at most τorthogonal pilot signals available for the UEs 106. For example, if acoherence interval comprises 200 samples and 50 UEs 106 are to be servedby the RBS 102, 50/200=¼ of the coherence interval is allocated to thepilot signals. Hence, further increasing the number of pilot signalsthat are available for the RBS 102 cannot be afforded.

At some point in time, the UEs 106 access the RBS 102, e.g., in order toestablish or join an uplink and/or downlink radio communication with thetelecommunications network 100. The “newcomer” UEs 106 is unfamiliar tothe RBS 102 in the sense that the RBS 102 does not have Channel StateInformation (CSI) for a MIMO channel dedicated to each of them and/or inthat a unique pilot signal among the set of pilot signals available forthe RBS 102 is not assigned to each of them. The expression newcomer UE,as used herein, encompasses any UE 106 for which the RBS 102 has no (orno valid) UE-specific channel estimation, UE-specific CSI and/orUE-specific precoder. Alternatively or in addition, the RBS has notassigned a dedicated pilot signal to the newcomer UE 106. Fornon-newcomer UEs, the CSI and precoding are re-estimated betweencoherence intervals using the dedicated pilot signals assigned by theRBS. Still alternatively or in addition, the expression newcomer UE, asused herein, encompasses any UE 106 that aims at accessing thetelecommunications network 102.

The newcomer UEs 106 is attempt joining the network, e.g., by using arandom access scheme. The pilot signals are selected, e.g. randomly orbased on an UE-identifier, from a set of all possible pilots or from asubset thereof. By way of example, the subset is reserved to first-timeaccess. The subset is complementary to a subset of protected pilotsignals that may not be used for first-time access, thus limitingcollisions to different newcomer UEs 106. The term “random”, as usedherein, encompasses pseudo-random, e.g., generated by means of adeterministic random bit generator implementable by a maximal linearfeedback shift registers. E.g., at a random time, the UE 106 sends anaccess burst including data, which is detected by the RBS 102. At firstglance, it may appear that the random access scheme can have the samecharacteristics in massive-MIMO systems as in conventional systems(e.g., Single-Input Single-Output transmissions or MIMO transmissionswith 2 spatial layers). However, there are a couple of major differencesrelated to the large number of transceiving antennas 104.

The access burst needs to be unique enough so that the RBS 102 is in acondition to meaningfully respond to the random access request. E.g.,the RBS 102 estimates the full CSI to the accessing UE 106 so thatclosed-loop beamforming is enabled. To this end, the access burstincludes a pilot signal that is exploited by the RBS 102 for channelestimation. If the pilot signal used by the UE 106 is unique, the UE 106sends the pilot signal periodically in each coherence interval. The RBS102 performs channel estimation based on the received pilot signal ineach coherence interval. If the pilot signal used in the access burst ofthe UE 106 is also used by another UE 106 that simultaneously sends itsaccess burst, the present technique is applicable to resolve theconflict.

By way of example, to perform downlink beamforming to the newcomer UE106 that is trying to access the telecommunications network 100, the RBS102 must be able to estimate the CSI from the access burst. This meansthat the newcomer UE 106 needs to have its own pilot signal that isorthogonal to every other pilot signal currently in use in the cell.This is a problem, because there are generally not enough symbolsequences defining the different pilot signals available. For example,out of 50 distinct pilot signals assignable within one cell served bythe RBS 102, 40 pilot signals are reserved to active (e.g.,RRC-connected) UEs 106 and the remaining 10 pilot signals are reservedfor random access (i.e., for use by newcomer UEs 106, e.g., RRC-idle UEs106).

If there are many newcomer UEs 106, there is a substantial probabilitythat (at least) two of them choose the same pilot signals among thosereserved for random access. If a pilot signal collision occurs, the RBS102 has no way of distinguishing the (at least) two UEs 106. All themore, the RBS 102 cannot identify the (at least) two UEs 106.

A conventional approach of resolving the conflict is for the at leasttwo UEs 106 to try again, e.g., at a random point in time and/or byselecting a random pilot signal (also referred to as “Aloha” approach).The conventional approach is inefficient in terms of delay andthroughput. Particularly when the system load is high, rare radiocapacity is occupied by access attempts. The present technique isapplicable to resolve the conflict more efficiently.

FIG. 2 shows a schematic block diagram of a device 200 for assigningpilot signals to user equipments that access a radio base station, e.g.,the radio base station of FIG. 1. The radio base station provides radioaccess to the user equipments via a plurality of transceiving antennas.The device 200 comprises a receiving unit 202 adapted to receive accessbursts from the user equipments at the radio base station within acoherence interval. The received access bursts include pilot signalsfrom the user equipments.

An estimating unit 205 of the device 200 is adapted to perform channelestimation based on the received access bursts for channelsdistinguished based on different pilot signals. A computing unit 206 ofthe device 200 is adapted to sum up, over the plurality of transceivingantennas, signal power as received at the radio base station for thechannels distinguished based on the different pilot signals. A channelanalysis unit 204 may implement both the estimating unit 205 and thecomputing unit 206.

If one of the different pilot signals is used by at least two of theuser equipments (e.g., among the user equipments from which the accessbursts are received in the coherence interval), the correspondingestimated channel is a multicast channel to the at least two userequipments. The received signal power computed for the multicast channelis a combined signal power received at the radio base station from theat least two user equipments.

A sending unit 208 is adapted to send a message on the broadcast channelto the at least two user equipments. The broadcast message is indicativeof the combined signal power. The broadcast message triggers the atleast two user equipments to set their own pilot signal depending on asignal power received at the respective one of the at least two userequipments in comparison with the combined signal power indicated in thebroadcast message.

The device 200 may be implemented at the RBS 102.

FIG. 3 shows a schematic block diagram of a device 300 for assigningpilot signals to user equipments that access a radio base station, e.g.,the user equipments of FIG. 1. The radio base station provides radioaccess to the user equipments via a plurality of transceiving antennas.

The device 300 comprises a sending unit 302 adapted to send an accessburst from a user equipment to the radio base station within a coherenceinterval. The access burst includes a pilot signal from the userequipment. The user equipment selects the pilot signal to be included inthe access burst.

A receiving unit 304 is adapted to receive a message from the radio basestation in response to the access burst. The receiving unit or any otherunit of the user equipment determines that the message is indicative ofa combined signal power, as received at the radio base station. Thedetermination implies that at least one further user equipment has sentthe same pilot signal in the coherence interval. The determinationfurther implies that the received message has been sent using amulticast channel from the radio base station to the user equipment andthe at least one further user equipment.

An estimating unit 307 of the device 300 is adapted to estimate achannel to the radio base station, e.g., by performing channelestimation based on the received multicast message. A computing unit 308of the device 300 is adapted compute a signal power as received at theuser equipment from the radio base station based on the estimatedchannel. Optionally, a channel analysis unit 306 implements both theestimating unit 307 and the computing unit 308.

A determining unit 310 is adapted to determine a pilot assigned to theuser equipment by comparing the computed received signal power with thecombined signal power.

In an exemplary implementation, the received message includes one ormore lists of pilot signals and the user equipment selects its own pilotsignal from the one or more lists. E.g., a list out of two or more listsor a position in the list is determined.

The device 300 may be implemented at one or more of the UEs 106.

FIG. 4 shows a flowchart for a method 400 of assigning pilot signals touser equipments accessing a radio base station, e.g., the radio basestation of FIG. 1. The radio base station provides radio access by meansof a plurality of transceiving antennas. The method 400 comprises a step402 of an access burst including initial pilot signals from the userequipments. At least two of the user equipments use the same initialpilot signal.

In an estimating step 404, a multicast channel to the at least two userequipments is estimated based on the received access burst including thesame initial pilot signal. In a computing step 406, a combined signalpower received at the radio base station is computed for the at leasttwo user equipments based on the estimated multicast channel. Thecomputation includes a summation over the plurality of transceivingantennas. In a sending step 408, a message is sent to the at least twouser equipments using the multicast channel. The message is indicativeof the combined signal power and assigns different pilot signals to theat least two user equipments. The assignment depends for each of the atleast two user equipments on the combined signal power and a signalpower received at the respective user equipment.

The method 400 may be implemented at the RBS 102, e.g. in the device200. The units 202, 205, 206 and 208 may perform the steps 402 to 408,respectively. The method 400 may be performed when the RBS 102 isaccessed using a colliding pilot signal.

FIG. 5 shows a flowchart for a method 500 of assigning pilot signals touser equipments accessing a radio base station, e.g., the userequipments of FIG. 1. The radio base station provides radio access bymeans of a plurality of transceiving antennas. The method 500 comprisesa step 502 of sending an access burst including an initial pilot signalfrom a user equipment to the radio base station. At least one other ofthe user equipments uses the same initial pilot signal. In a step 504, amulticast message is received from the radio base station. The multicastmessage is indicative of a combined signal power received at the radiobase station from the user equipment and the at least one other userequipment using the same initial pilot signal. In a step 506, a channelto the radio base station is estimated. In a step 508, a signal powerreceived at the user equipment is computed based on the estimatedchannel. In a step 510, the assigned pilot signal for the user equipmentis determined. The assignment depends on the combined signal powerindicated by the multicast message and the signal power received at theuser equipment.

The method 500 may be implemented at one or more of the UEs 106, e.g.,in the device 300. The units 302, 304, 307, 308 and 310 may perform thesteps 502 to 510, respectively. The method 500 may be performed when twoor more UEs 106 access the RBS 102 using a colliding pilot signal.

FIG. 6 shows a flowchart for an implementation of the method 400. Stepscorresponding to those of FIG. 4 are indicated by correspondingreference signs. The telecommunications network 100 comprises at leastone RBS 102 and at least two UEs 106, denoted by A and B. The RBS 102detects a collision between two random access bursts from the UEs A andB, respectively, in a step 403. The step 403 is optionally implementedas a substep of the step 402. In the event of a pilot signal collisiondetected in the access bursts, the RBS 102 estimates a channel, g, basedon the colliding pilot signals the step 404. Herein, g is a vectorincluding the complex-valued channel estimates for each of the radiobase station antennas 104. As the estimates are based on the collidingpilot signals, g includes contributions from each of the correspondingUEs 106 (cf. Eq. (3) below).

The RBS 102 computes in the step 406 the received signal power, ∥g∥², inthe event of a detected collision. The received signal power ∥g∥²includes a summation over the plurality of M transceiving antennas 104.The received signal power ∥g∥² is summed, e.g., averaged, over theplurality of transceiving antennas 104. The averaged received signalpower, ∥g∥²/M, in the uplink transmission is an exemplary estimator forthe average of individually received signal powers at the UEs A and B ina downlink transmission within the same coherence interval:

p _(A) p _(A) +p _(E)β_(E).

The RBS 102, in the event of a detected collision, selects two distinctsets X and Y of pilot signals x_(i) and y_(t), respectively, from apredetermined pilot codebook (e.g., in the step 408). The RBS 102multicasts the multicast message (e.g., a control packet) withbeamforming along the multicast channel g, in the event of a detectedcollision, in the step 408. The message includes the estimate ofp_(A)β_(A)+p_(B)β_(B), an instruction that the stronger one of the UEs Aand B has to use a pilot signal x (e.g., randomly selected) from the setX, and that the weaker one of the UEs A and B has to use a pilot signalY (e.g., randomly selected) from the set Y. In an exemplaryimplementation, the respective UE determines being the stronger orweaker one by comparing twice its channel gain with the combined channelgain.

As a result of the steps 502 to 508 of the method 500, the UEs A and Bhave computed their individually received signal powers p_(A)β_(A) andp_(B)β_(B), respectively, in a downlink transmission within the samecoherence interval during which the access burst is sent. According tothe step 510, each of the UEs A and B determines whether it is thestronger one of the UEs A and B or the weaker one of the UEs A and B.

The UEs A and B re-access the RBS 102 using new pilot signals accordingto the instruction from the RBS 102. In a corresponding step 410, theRBS 102 receives the further access bursts including collision-freepilot signals from the UEs A and B, respectively.

Background information for massive-MIMO operation is provided in detail,for example, in tutorials by E. G. Larsson et al. “Massive MIMO for nextgeneration wireless systems”, IEEE Commun. Mag., vol. 52, no. 2, pp.186-195, February 2014; and by F. Rusek et al. in “Scaling up MIMO:Opportunities and challenges with very large arrays”, IEEE SignalProcess. Mag., vol. 30, no. 1, pp. 40-60, January 2013.

Further technical details about beamforming and CSI estimation areprovided, for example, by H. Q. Ngo et al. in “Energy and spectralefficiency of very large multiuser MIMO systems”, IEEE Trans. Commun.,vol. 61, pp. 1436-1449, April 2013; by T. L. Marzetta in “Noncooperativecellular wireless with unlimited numbers of base station antennas”, IEEETrans. Wireless Commun., vol. 9, no. 11, pp. 3590-3600, November 2010;by E. Björnson et al. in “Massive MIMO systems with non-ideal hardware:Energy efficiency, estimation, and capacity limits”, IEEE Trans. Inf.Theory, vol. 60, no. 11, pp. 7112-7139, November 2014; and by E.Björnson et al. in “Optimizing multi-cell massive MIMO for spectralefficiency: How many users should be scheduled?”, in Proc. IEEE GlobalConf. on Signal and Inf. Process. (GlobalSIP), 2014.

An embodiment of the technique is described in more detailed withexemplary explanations for numerical relations useful for implementingthe evaluation steps 404, 506 and the computing steps 406, 508.Well-known properties of massive-MIMO, i.e., M>>1 (e.g., M≧9, M≧16 orM≧25) include channel hardening and near-orthogonality (also referred toas asymptotic orthogonality) between channels of the UEs.

FIG. 7 schematically illustrates a massive-MIMO communication withTime-Division Duplex (TDD) operation. FIG. 7 shows (in its top-left) atime-frequency grid including a frame 700 for bidirectional radiotransmission from and to the RBS 102. A spectral window 702 is allocatedto the UEs 106 for a Time-Division Duplex (TDD) communication with theRBS 102.

The time-frequency plane is partitioned into frames 700, which in turnare partitioned into coherence intervals 706. In combination with aspectral range 708, the coherence intervals are also referred to asResource Blocks (RBs) 704. For a spectral width B_(C) indicated atreference sign 708 for the RB 704, the coherence interval 706 comprisesB_(C)·T_(C) samples in the time domain.

During one coherence interval 706, the channel between each transmitantenna (e.g., the antennas 104 in case of a DL transmission) and eachreceive antenna (e.g., the antennas 104 in case of an UL transmission)substantially acts as a scaling with a complex number g_(m,k) for m=1, .. . , M and k=1, . . . , K. In other words, electromagnetic radiopropagation from each of the transmit antennas to each of the receiveantennas is represented by a channel gain and a channel phase. Themethod can be performed within each coherence interval 706. The methodcan be performed independently for each coherence interval 706.

Channel hardening relates to the observation that the channel impulseresponses g_(k) of a massive-MIMO transmission “harden”, at least inmost propagation environments. This means, more exactly, that if g_(k)denotes an M-vector comprising the complex-valued gains g_(m,k) (form=1, . . . , M) from the M RBS antennas 104 to the k-th UE 106 in agiven coherence interval, then when M is large

$\begin{matrix}{{{\frac{1}{M}{g_{k}}^{2}} \approx \beta_{k}},} & (1)\end{matrix}$

wherein β_(k) is a scalar number that does not depend on aninstantiation of fast fading but only on parameters that vary slowly,e.g., path loss and shadow fading.

Equation (1) is true in many diverse propagation environments. Forexample, in a propagation environment with independently and identicallydistributed (i.i.d.) Rayleigh fading, Eq. (1) holds because of the lawof large numbers. The sum of M independent exponentially distributedrandom numbers, normalized by M, converges to its expected value whenM→∞. Line-of-sight propagation according to H. Q. Ngo et al. in “Aspectsof favorable propagation in massive MIMO”, in Proc. European Wireless,2014, is a further example for a propagation environment in which Eq.(1) applies.

There are also very special propagation environments without channelhardening (e.g., the keyhole channel observed by P. Almers et al. in“Keyhole effect in MIMO wireless channels: Measurements and theory”,IEEE Trans. Wireless Commun., vol. 12, no. 5, pp. 3596-3604, December2006, and analyzed by H. Q. Ngo and E. G. Larsson in “Blind estimationof effective downlink channel gains in massive MIMO” in Proc. IEEE Int.Conf. on Acoustics, Speech, and Signal Process. (ICASSP), preprintidentifier urn:nbn:se:liu:diva-112759). Consequently, the proposedexplanations and relations should be thought of as a way ofsignificantly enhancing a random access mechanism in at least manypropagation environments.

Another consequence of channel hardening is that channels to differentUEs 106 are nearly orthogonal if M is large. Denoting the channels fromthe RBS 102 to UEs A and B by g_(A) and g_(B), respectively, thenear-orthogonality means that

$\begin{matrix}{{\frac{1}{M}g_{A}^{H}g_{B}} \approx 0.} & (2)\end{matrix}$

The multicasting channel to the UEs A and B used by the unit 208 for thestep 408 includes both a channel component g_(A) and a channel componentg_(B). Since the two UEs A and B transmit the same pilot signal in theiraccess burst 710, the RBS 102 obtains a channel estimate

g=√{square root over (p _(A))}g _(A)+√{square root over (p _(B))}g _(B)+ñ  (3)

during the channel estimation step 404, wherein g_(A) and g_(B) are theindividual channels to UEs A and B for the coherence interval 706. Atthis point, the individual channels g_(A) and g_(B) are not known to theRBS 102.

Power-control coefficients for the UEs A and B are denoted by p_(A) andp_(B), respectively. A typical massive-MIMO implementation assumes thatp_(A) and p_(B) are both put to their maximal value in the initial pilottransmission 712 (i.e., in the access burst 710), to achieve highestimation accuracy. More generally, an exemplary implementation assumesthat p_(A)=p_(B). The present technique is, however, applicable for anynon-zero p_(A) and p_(B).

The vector ñ is an estimation error (e.g., resulting from thermal noiseand, potentially, interference from distant cells). Hence, the RBS 102cannot determine g_(A) and g_(B) individually. The RBS 102 measures thepower-controlled sum

√{square root over (p _(A))}g _(A)+√{square root over (p _(B))}g _(B).

Since the channels g_(A) and g_(B) are near-orthogonal in massive-MIMO,as described with reference to Eq. (2), the channels do not cancel outone another in above sum. Thus, the RBS 102 obtains a useful CSI for themulticast channel to both UEs A and B.

This phenomenon is exploited for the purpose of multicasting in the step408 for sending the same message 716 to the colliding UEs 106.Technically, the message 716 is beamformed along g (which is known tothe RBS 102 as a result of the step 404), so that the message 716reaches both UEs A and B.

A pilot collision occurs between two newcomer UEs A and B, i.e. the UEsA and B use the same pilot signal 712 in their overlaying access bursts710 within the coherence interval 706. The RBS 102 obtains in the step404 the estimate g=√{square root over (p_(A))}g_(A)+√{square root over(p_(B))}g_(B)+ñ according to Eq. (3) for the multicast channel.

The RBS 102 cannot decode UE-specific information 714 contained in thereceived access burst 710. Hence, the RBS 102 cannot distinguish oridentify the newcomers UEs A and B. However, the RBS 102 is able todetermine, with reasonable certainty, that a collision has occurred. Forexample, the RBS 102 determines the presence of a pilot signal collisionin the step 403 by concluding that decoding of the access burst 710failed although the received (combined) pilot signal power ∥g∥² is abovea threshold that is likely to give successful decoding when there are nocollisions.

Having detected the collision, the RBS 102 sends in the step 408 themessage 716, which is received at both UEs A and B, by beamforming alongthe channel estimate g.

The RBS 102 indicates in the message 716 to the two UEs A and B toselect pilot signals from the sets X and Y, instead of the pilot signal712 used in the access burst 710. The sets X and Y have no pilot signalin common. Since the RBS 102 cannot identify or distinguish the UEs Aand B, the RBS 102 cannot address them individually. Moreover, the UEs Aand B cannot identify or address the respectively other UE using thesame pilot signal 712 in the access burst 710.

In a reduced implementation, the pilot-collision problem isstatistically solved. The RBS 102 instructs in the message 716 to selectpilot signals from the sets X and Y, instead of the pilot signal 712used in the access burst 710. In the reduced implementation, the UEs Aand B randomly select a pilot signal from the set X or Y. With 50%probability, the UEs A and B select the same set. Thus, a furthercollision will occur in the retransmission of the access burst withprobability

${\frac{0.25}{X} + \frac{0.25}{Y}},$

wherein |X| and |Y| are the number of pilots in each set.

For example, |X|=|Y|=1 leads to 50% collision probability in theretransmission and |X|=|Y|=5 leads to 10% collision probability in theretransmission.

In an advanced implementation, the message 716 instructs the UEs A and Bto determine different pilot signals based on the indicated combinedsignal power and the individually measured signal power. Since

${{\frac{1}{M}{g_{A}}^{2}} \approx {\beta_{A}\mspace{14mu} {and}\mspace{14mu} \frac{1}{M}{g_{B}}^{2}} \approx \beta_{B}},{{{and}\mspace{14mu} \frac{1}{M}g_{A}^{H}g_{B}} \approx 0}$

(by the channel hardening explained above), the RBS 102 computes in thestep 406 the combined signal power

p _(A)β_(A) +p _(B)β_(B)

by computing

${\frac{1}{M}{\hat{g}}^{2}} \approx {{\frac{p_{A}}{M}{g_{A}}^{2}} + {\frac{p_{B}}{M}{g_{B}}^{2}}} \approx {{p_{A}\beta_{A}} + {p_{B}\beta_{B}}}$

assuming that the noise is relatively weak.

The first and second equation signs in Eq. (4) exploit thenear-orthogonality and the channel hardening according to Eqs. (2) and(1), respectively. In other words, the massive-MIMO implementation ofthe RBS 102 is enabled to compute, with reasonable accuracy, thecombined signal power p_(A)β_(A)+p_(B)β_(B) according to Eq. (4) in thestep 406.

The UEs A and B learn their own channel gain β by listening to themulticast transmission 716 (or by listening to a beacon frequency, forthat matter). So UE A knows β_(A) (and p_(A)) and UE B knows β_(B) (andp_(B)). It is not necessary that the UEs A and B know each others' β orpower level p.

In order to prevent a pilot signal collision in the retransmission, theRBS 102 and the colliding UEs 106 perform the steps 408 and 504 to 510,respectively. The RBS 102 multicasts by beamforming along g in the step408 the message 716

indicative of the combined power (p_(A)β_(A)+p_(B)β_(B)) being equal to[the value computed in step 406]; and

instructing to re-access using a pilot signal from the set X if the UEhas the “strong” channel, otherwise using a pilot signal from the set Y.

The UE A receives the multicast message 716. Based on the message 716,the UE A is informed of the combined power p_(A)β_(A)+p_(B)β_(B). SinceUE A already knows its individual signal power contribution p_(A)β_(A)as a result of the step 508, the UE A computes p_(B)β_(B) and determineswhether

p _(A)β_(A) >p _(B)β_(B)

or vice versa in an implementation of the step 510. Ifp_(A)β_(A)>p_(B)β_(B), the UE A concludes that it has the “strong”channel and selects a pilot signal x randomly from the set X accordingto the instruction in the message 716. Otherwise, the UE A concludesthat it has the “weak” channel and selects a pilot signal y randomlyfrom the set Y.

Alternatively, the UE A determines whether

2p _(A)β_(A) >p _(A)β_(A) +p _(B)β_(B),

i.e., UE A is the stronger one, or

2p _(A)β_(A) <p _(A)β_(A) +p _(B)β_(B),

i.e., the UE A is the weaker one.

Similarly, UE B determines in the step 510 whether

p_(A)β_(A)>p_(B)β_(B)

or vice versa. If p_(A)β_(A)>p_(B)β_(B), UE B concludes that it has the“weak” channel and selects a pilot signal y randomly from Y. Otherwise,the UE B concludes that it has the “strong” channel and selects a pilotsignal x randomly from the set X.

Alternatively, the UE B determines whether

2p _(B)β_(B) >p _(A)β_(A) +p _(B)β_(B),

i.e., UE B is the stronger one, or

2p _(B)β_(B) <p _(A)β_(A) +p _(B)β_(B),

i.e., the UE B is the weaker one.

So in the retransmission, the UEs A and B are guaranteed to selectdifferent pilot signals (among the permissible sets X and Y,respectively) and hence succeed without collision. If the pilot signalsets X and Y are not open for random access, there is also no risk forcollision with further newcomer UEs 106.

In any of above exemplary implementations, the channel gain β for thedifferent UEs 106 in a cell of the RBS 102 may largely vary. E.g., witha path loss exponent equal to four, the difference in the channel gain βis about a factor 10⁴ between two UEs 106 which distance to the RBS 102differs by a factor of ten. Therefore, two colliding newcomers UEs 106typically have very different values for the channel gain β.

In an embodiment of the technique, open-loop power-control is utilizedto set the transmission power p for the pilot signal 712 as a functionof the channel gain β. However, substantial variations in p·β are likelyto exist in the cell, unless a very stringent power-control policy isused so that p proportional to 1/β. Often, a dynamic range of poweramplifiers does not allow for such a power-control policy.

If there is a substantial probability that p_(A)β_(A)≈p_(B)β_(B), e.g.,due to propagation conditions and/or power control, there is a risk thatboth UEs A and B determine in the step 510 themselves as having the“strong” channel or the “weak” channel.

Optionally, e.g., in this case, the UEs A and B further analyze theirreceived signals 716 in the downlink.

E.g., UE A receives

g _(A) ^(H) g≈√{square root over (p _(A))}∥g _(A)∥²+√{square root over(p _(B))}g _(A) ^(H) g _(B)

and UE B receives

g _(B) ^(H) g≈√{square root over (p _(B))}∥g _(A)∥²+√{square root over(p _(A))}g _(B) ^(H) g _(A).

The phase of the second term is the same but complex-conjugated for thetwo UEs A and B. The phase difference is optionally used to separate thetwo UEs A and B to reduce the collision probability in theretransmission.

Each of the distinct pilot signal sets X and Y may contain a singlepilot signal or multiple pilot signals. Having multiple pilots reduces aresidual risk for a new collisions in the retransmission, e.g., in theevent that one of the colliding UEs is not able to correctly determinewhether it has the “strong” or “weak” channel. In other words, anembodiment combines received signal power-based solution with thestatistical solution.

With some probability, three or more newcomer UEs 106 could accidentallyselect the same pilot signal. However, the probability of such acollision between three UEs 106 is vastly smaller than the probabilityof a collision between two UEs 106. Some embodiments are extended toresolve higher-order collisions, e.g., by iteratively applying the abovetechnique to two out of the at least three colliding UEs and/or byassigning pilot signals in the order of the signal power contribution.Furthermore, combination with the statistical solution (|X|>1 and |Y|>1)allows avoiding collisions in the retransmission.

The lower half of FIG. 7 schematically illustrates a communication forimplementing the technique.

Each coherence interval 706 is partitioned into an uplink part for theaccess burst 710 and a downlink part for the message 716. In the uplinkphase 710 (illustrated at the bottom-left), the at least two newcomerUEs 106 send their initial pilot signal 712. The active UEs 106 sendtheir assigned pilot signal 712.

The RBS 102 estimates the channel based on the mutually orthogonal pilotsignals 712 for each of the UEs 106. A UE-specific CSI is a result ofthe channel estimation for each non-colliding UE 106. The multicast CSIis a result of the channel estimation for colliding UE 106 using thesame pilot signal according to the step 404. The UE 106 also send uplinkdata 714 in the coherence interval 706.

In the downlink phase 716 (illustrated at the bottom-right), the RBS 102uses the channel estimates (illustrated at reference signs 718) obtainedbased on the uplink pilot signal to beamform data in a closed-loop modeto the UEs 106. Large spatial multiplexing gains are being harvested byserving many UEs 106 simultaneously in each coherence interval 706.

The multicasting step 408 optionally sends the message 716 at the sametime-frequency resource as other beamforming transmissions 718, sincethe channel hardening property of Eq. (2) implies that the multicastingtransmission 716 and the UE-specific transmissions 718 are mutuallynearly orthogonal. An even higher degree of orthogonality is achieved bychanging the multicast beamforming from g to S⁻¹g, wherein S⁻¹ is afull-rank matrix that shapes the multicasting to suppress interferencein certain spatial dimensions. E.g., strong eigen-directions of S aresuppressed and weak eigen-directions are not suppressed.

FIG. 8 schematically illustrates a network node 800 for assigning pilotsignals to user equipments accessing a radio base station that providesradio access. The network node 800 comprises functional modules 802 to808 to realize the steps 402 to 408, respectively. In an embodiment ofthe network node, e.g., the RBS 102, the modules are implemented by acomputer program running on a processor of the network node 800.

FIG. 9 schematically illustrates a mobile terminal 900 for assigningpilot signals to user equipments accessing a radio base station thatprovides radio access. The network node 800 comprises functional modules902 to 910 to realize the steps 502 to 510, respectively. In anembodiment of the mobile terminal, e.g., the UE 106, the modules areimplemented by a computer program running on a processor of the mobileterminal 900.

As has become apparent from above exemplary embodiments, the techniqueis applicable to a large number of phase-coherently operated antennas,e.g., in any LTE telecommunications standard and beyond LTE.

Embodiments of the presented technique reduce the extra amount oftime-frequency resources needed for random access and/or reduce thelatency of the random access in at least some embodiments. Thisadvantage is valuable, e.g., in the context of machine-to-machinecommunications. The technique is optionally implemented in atelecommunications network for machine-to-machine communications with alarge number of sleeping user equipments that access the radio basestation with little delay and low energy expenditure.

Many advantages of the present invention will be fully understood fromthe foregoing description, and it will be apparent that various changesmay be made in the form, construction and arrangement of the units anddevices without departing from the scope of the invention and/or withoutsacrificing all of its advantages. Since the invention can be varied inmany ways, it will be recognized that the invention should be limitedonly by the scope of the following claims.

What is claimed is:
 1. A method of assigning pilot signals to userequipments accessing a radio base station that provides radio access,the method comprising: receiving an access burst from the userequipments at the radio base station, the access burst including initialpilot signals, wherein at least two of the user equipmentssimultaneously apply the same initial pilot signal; estimating, at theradio base station, a multicast channel to the at least two userequipments based on the received access burst; and sending, from theradio base station, a message to the at least two user equipments usingthe multicast channel, wherein the message assigns different pilotsignals to the at least two user equipments; wherein the assignment ofthe different pilot signals, for each of the at least two userequipments, depends on a signal power received at the respective userequipment.
 2. The method of claim 1, wherein the signal power receivedat the respective user equipment is computed based on either: themessage as received at the respective user equipment; or a broadcastmessage from the radio base station.
 3. The method of claim 1, whereinboth the access burst is received and the message is sent within onecoherence interval.
 4. The method of claim 1, wherein the at least twouser equipments include a first user equipment and a second userequipment different from the first user equipment; wherein the messageassigns a first pilot signal to the first user equipment if twice thesignal power received at the first user equipment is greater than acombined signal power received at the radio base station for the atleast two user equipments; and wherein the message assigns a secondpilot signal, different from the first pilot signal, to the second userequipment if twice the signal power received at the second userequipment is less than the combined signal power.
 5. The method of claim4, wherein the message assigns a first set of pilot signals includingthe first pilot signal to the first user equipment and a second set ofpilot signals disjoint from the first set and including the second pilotsignal to the second user equipment; and wherein the first and seconduser equipments select the first and second pilot signals out of thefirst and second sets, respectively.
 6. The method of claim 1, whereinthe initial pilot signals from the at least two user equipments overlapin time and frequency.
 7. The method of claim 1, wherein the radio basestation determines that the at least two user equipments applysimultaneously the same initial pilot signal if the received accessburst is not decodable and a signal strength of the access burst exceedsa threshold value.
 8. The method of claim 1, wherein the signal powerreceived at the respective user equipment includes a product of linkattenuation (β) and transmit power (p); and wherein the link attenuationis an attenuation for a link between the radio base station and therespective user equipment.
 9. The method of claim 1, further comprising:receiving, from each of the at least two user equipments, a furtheraccess burst including the respectively assigned pilot signal; andestimating a channel to each of the at least two user equipments basedon the received further access burst.
 10. The method of claim 1, whereinthe radio access includes a time division duplex transmission; andwherein the pilot signals are configured for reverse channel estimation.11. A computer program product stored in a non-transitory computerreadable medium for assigning pilot signals to user equipments accessinga radio base station that provides radio access, the computer programproduct comprising software instructions which, when run on one or moreprocessing circuits of the user radio base station, causes the radiobase station to: receive an access burst from the user equipments at theradio base station, the access burst including initial pilot signals,wherein at least two of the user equipments simultaneously apply thesame initial pilot signal; estimate, at the radio base station, amulticast channel to the at least two user equipments based on thereceived access burst; and send, from the radio base station, a messageto the at least two user equipments using the multicast channel, whereinthe message assigns different pilot signals to the at least two userequipments; wherein the assignment of the different pilot signals, foreach of the at least two user equipments, depends on a signal powerreceived at the respective user equipment.
 12. A network node forassigning pilot signals to user equipments accessing a radio basestation that provides radio access, the network node comprising: anaccess burst reception circuit configured to receive an access burstfrom the user equipments at the radio base station, the access burstincluding initial pilot signals, wherein at least two of the userequipments simultaneously apply the same initial pilot signal; a channelestimation circuit configured to estimate a multicast channel to the atleast two user equipments based on the received access burst; and amessage send circuit configured to send a message to the at least twouser equipments using the multicast channel, wherein the message assignsdifferent pilot signals to the at least two user equipments; wherein theassignment of the different pilot signals, for each of the at least twouser equipments, depends on a signal power received at the respectiveuser equipment.
 13. A device comprised in a radio base station forassigning pilot signals to user equipments accessing a radio basestation that provides radio access, the device comprising: a receivingcircuit configured to receive an access burst from the user equipmentsat the radio base station, the access burst including initial pilotsignals, wherein at least two of the user equipments simultaneouslyapply the same initial pilot signal; an estimation circuit configured toestimate a multicast channel to the at least two user equipments basedon the received access burst; and a sending circuit configured to send amessage to the at least two user equipments using the multicast channel,wherein the message assigns different pilot signals to the at least twouser equipments; wherein the assignment of the different pilot signals,for each of the at least two user equipments, depends on a signal powerreceived at the respective user equipment.
 14. A method of assigningpilot signals to user equipments accessing a radio base station thatprovides radio access, the method comprising: sending an access burstfrom a user equipment to the radio base station, the access burstincluding an initial pilot signal, wherein at least one other of theuser equipments applies simultaneously the same initial pilot signal;receiving, at the user equipment, a multicast message from the radiobase station; estimating, at the user equipment, a channel to the radiobase station; and determining, at the user equipment, a pilot signalassigned to the user equipment, wherein the assignment depends on themulticast message and the estimated channel.
 15. The method of claim 14,wherein the estimating the channel comprises estimating the channelbased on the multicast message as received at the user equipment orbased on a broadcast message from the radio base station.
 16. The methodof claim 14, wherein a first pilot signal is assigned to the userequipment if twice the signal power received at the user equipment isgreater than the combined signal power; and wherein a second pilotsignal, different from the first pilot signal, is assigned to the userequipment if twice the signal power received at the user equipment isless than the combined signal power.
 17. The method of claim 14, furthercomprising sending a further access burst including the assigned pilotsignal to the radio base station.
 18. A computer program product storedin a non-transitory computer readable medium for controlling a userequipment, the user equipment being one of a plurality of userequipments accessing a radio base station that provides radio access,the computer program product comprising software instructions which,when run on one or more processing circuits of the user equipment,causes the user equipment to: send an access burst from the userequipment to the radio base station, the access burst including aninitial pilot signal, wherein at least one other of the user equipmentsapplies simultaneously the same initial pilot signal; receive, at theuser equipment, a multicast message from the radio base station;estimate, at the user equipment, a channel to the radio base station;and determine, at the user equipment, a pilot signal assigned to theuser equipment, wherein the assignment depends on the multicast messageand the estimated channel.
 19. A mobile terminal, comprising: an accessburst sending circuit configured to send an access burst to the radiobase station, the access burst including an initial pilot signal,wherein at least one other of the user equipments applies simultaneouslythe same initial pilot signal; a message reception circuit configured toreceive a multicast message from the radio base station; a channelestimation circuit configured to estimate a channel to the radio basestation; and a pilot signal determination circuit configured todetermine a pilot signal assigned to the user equipment, wherein theassignment depends on the multicast message and the estimated channel.20. A device comprised in a user equipment accessing a radio basestation for assisting in the assignment of pilot signals to userequipments accessing the radio base station that provides radio access,the device comprising: a sending circuit configured to send an accessburst to the radio base station, the access burst including an initialpilot signal, wherein at least one other of the user equipments appliessimultaneously the same initial pilot signal; a receiving circuitconfigured to receive a multicast message from the radio base station;an estimating circuit configured to estimate a channel to the radio basestation; and a determining circuit configured to determine a pilotsignal assigned to the user equipment, wherein the assignment depends onthe multicast message and the estimated channel.